Porous carbons are versatile materials because of their extended surface area and microporous structure. They find application as filters, membranes, sorbents and catalyst supports for materials in both gas and liquid phases. Porous carbons also find use in the treatment and remediation of domestic and industrial wastewaters. See, for example, Bansal, R. C., J.-B. Donnet and F. Stoeckli (1988), Active Carbon, Marcel Dekker: New York. In addition, carbon is extensively used as an electrode material as described in Kinoshita, K. (1988), Carbon: Electrochemical and Physicochemical Properties, John Wiley & Sons: New York.
The properties and suitability of porous carbons toward specific applications is dictated in large measure by the precise character of the porosity of the carbon. In particular, the size, shape and the distribution in size of pores heavily influence the characteristics of the porous material and its suitability for a given application. Pore structures can assume a multitude of shapes and configurations each varying in shape, depth and width. The interior of a pore can differ markedly from the cross-section presented on the external surface of the porous material with respect to both shape and size. Some exemplary pore structures include tubular capillaries, open or closed capillaries, ink-bottle-shaped capillaries, open slit-shaped capillaries and spheroidal pores. The way in which a pore is measured depends upon the shape of the pore: cylindrical pores are measured according to their diameter while slit-shaped pores are measured across their shortest dimension. Pores having characteristic dimensions (diameter or width) less than 2 nm are defined as micropores, those greater than 50 nm are macropores and those between 2 and 50 m are considered to be mesopores. The definition and size range encompassed by the term mesopore is well-accepted within the art and conforms to the convention adopted by the IUPAC; see for example, Bansal, R. C., J.-B. Donnet and F. Stoeckli, Active Carbon (1988) pp. 119-163.
Experimentally, porosity data may be acquired from nitrogen isotherms and pore size distribution plots. A nitrogen isotherm is a plot of the volume of nitrogen gas adsorbed and desorbed as a function of relative pressure measured at constant temperature. As described in Adsorption Surface Area and Porosity by S. Gregg and K. Sing, Academic Press: New York, 1982, these isotherms can be categorized into one of four types based on the shape of the plot. Type I isotherms feature a nearly horizontal plateau and little or no difference between the adsorption and desorption traces. However, Type IV isotherms are characterized by two inflection points and a difference in the adsorption and desorption isotherms, known as hysteresis. Type I isotherms are associated with microporous materials while Type IV isotherms are associated with the presence of mesopores. Thus, hysteresis is diagnostic of mesoporous materials.
It has been found that porous carbons characterized by a high proportion of mesopores are preferred for certain liquid-phase applications because of a unique profile of characteristics. For example, mesopores are more easily filled by electrolytes, which enhances their utility as electrodes when the carbon must be in intimate contact with a liquid electrolyte. The presence of a hysteresis loop in the nitrogen adsorption/desorption isotherm may be indicative of a mesoporous material. Mesoporosity can also be diagnosed via a pore size distribution plot that can be obtained according to the method of Barrett et al., J. Am. Chem. Soc. (1951), vol. 73, pp. 373-380, incorporated herein by reference.
Methods of manufacturing porous carbons influence the character and distribution of pores. Most methods give a distribution of pore sizes including micro-, meso-, and macro-pores. Those trying to make mesoporous carbons have attempted to change this distribution to increase the proportion of mesopores.
It has been known for some time that carbohydrates can be dehydrated to a carbon product. For example, carbon can be produced by the addition of concentrated sulfuric acid to common table sugar (sucrose) to produce carbon. High temperature treatment of such polymerized carbons under an inert atmosphere at temperatures in excess of 900° C. produces porous carbons of surface areas approaching 10 m2/g.
Some in the art have attempted to make mesoporous carbons in the presence of pore-forming materials and carbon precursors. For example, U.S. Pat. No. 6,024,899 to Peng et al. relates to making mesoporous carbon by combining a carbon precursor and pore-forming materials, wherein a pore former is preferably a thermoplastic material, e.g. polyvinylbutyrals (PVB), polyethylene glycols (PEG), heavy petroleum fractions and/or coal liquids.
U.S. Pat. No. 6,279,293 to Bell et al. discloses a mesoporous material prepared by polymerizing a resorcinol/formaldehyde (RF) system from an aqueous solution containing resorcinol, formaldehyde and a surfactant capable of stabilizing the electrostatic interactions between the monomer and surfactant. The surfactant may be cationic, anionic or nonionic with suitable surfactants including cetyltrimethylammonium chloride and cetyltrimethylammonium bromide, sodium dodecylbenzenesulfonic acid and sodium bis(2-ethylhexyl)sulfosuccinate, and Brij 30.
According to Jun et al. (2000) “Synthesis of New, Nanoporous Carbon with Hexagonally Ordered Mesostructure,” J. Am. Chem. Soc. Vol. 122, pp. 10712-10713 and the references contained therein, the synthesis of ordered nanoporous carbon materials was carried out using SBA-15, sucrose, and sulfuric acid wherein SBA-15 is an ordered mesoporous silica molecular sieve consisting of an hexagonal arrangement of cylindrical mesoporous tubes 9 nm in diameter.
Despite these advances within the art, research continues toward discovering and developing methods of making high surface area, mesoporous carbons that are: inexpensive, easy to implement and amenable to reliable duplication. In addition, there remains a need for applying such methods to the manufacture of select articles such as electrodes for ultracapacitors and capacitive deionization technology (CDT). It is anticipated that the mesoporous, high surface area carbons made from the present method will find particular use in applications that demand careful control over the number of mesopores, such as certain liquid phase and catalytic applications.
Water can be rendered undrinkable by virtue of dissolved salts, dirt or microorganisms. Treatment of brackish waters has heretofore primarily been carried out by a process of reverse osmosis (RO). Reverse osmosis can be understood if one considers two volumes of solution separated by a membrane through which solvent can pass but dissolved solutes cannot. Solvent will flow across the membrane from low to high solute concentration in a process described as osmosis. The pressure needed to counter the flow of solvent from low to high solute concentration is known as the osmotic pressure. Pressures in excess of the osmotic pressure may be used to reverse the flow of solvent such that solvent will flow from high to low solute concentration. It is upon this principle that reverse osmosis obtains pure water from brine. Reverse osmosis on a practicable scale is costly due to the large amount of energy required to generate sufficient pressure to overcome the osmotic pressure across a membrane at significant brine volumes. See, for example, Farmer et al. (1996), “Capacitive Deionization of NaCl and NaNO3 Solutions with Carbon Aerogel Electrodes,” J. Electrochem. Soc. 143, 159-169.
Alternatively, capacitive deionization technology (CDT) can be used to purify undrinkable water by passage of brackish water through a charged capacitor consisting of pairs of porous parallel electrodes maintained at a given potential difference. Dissolved salts, and microorganisms present in undrinkable water as charged species are attracted and bound to oppositely charged electrodes. The electrodes are used until they become saturated thereby requiring regeneration. Regeneration comprises removal of the applied potential and concomitant flushing to allow trapped ions and charged particles to migrate from the electrodes; contaminants are carried away as a concentrated brine stream. The energy required to operate a CDT system is substantially less than that required to drive reverse osmosis for an equivalent volume of liquid. CDT is therefore significantly less expensive to operate than reverse osmosis (RO). Broad adoption of CDT has been heretofore prevented by the high cost of manufacturing CDT electrodes which exceeds the capital cost of competing reverse osmosis systems. Others in the art have previously made CDT electrodes by a process comprising impregnating a carbon paper support with an aqueous resorcinol-formaldehyde solution, polymerizing the solution to obtain the resorcinol-formaldehyde resin impregnated upon the support, extracting the solvent from the resin/support, and pyrolyzing the resin/support to a carbon aerogel electrode. This process is deficient in more than one respect. It is an expensive process due to the relatively high cost of the resorcinol starting material and the high costs associated with the extraction step, said extraction employing, for example, supercritical carbon dioxide. Moreover, the electrodes obtained from the process are characterized by a relatively low surface area and therefore low capacity. The low capacity of the electrodes requires that a plurality of electrodes be used for an effective CDT system.
Another application for mesoporous carbons are as electrodes for ultracapacitors. Ultracapacitors based on double-layer capacitance store energy in a polarized liquid layer only a few angstroms thick at the interface between an ionically conducting electrolyte solution and an electronically conducting electrode. The separation of charge in the ionic species at the interface (called a double layer) produces a standing electric field. If other factors are equal, the larger the electrode surface area the more charge can be stored. In addition, because no chemical reactions take place during the charge/discharge cycle, these devices can be cycled many times without degradation.
There are two major categories of electrolytes for double layer ultracapacitor devices, aqueous and organic, each of which has their own set of advantages and disadvantages. Aqueous electrolytes such as potassium hydroxide and sulfuric acid have low resistance and can be charged and discharged very quickly, making them suitable for high power applications. However, they can only be cycled through a potential range of about one volt due to the voltage limits of aqueous electrolytes; this sharply limits their energy storage density (which is proportional to voltage squared). Organic electrolytes, such as NEt4BF4 dissolved in propylene carbonate or acetonitrile, have much higher decomposition voltages (up to three volts) and therefore have much greater energy storage densities (in theory, by a factor of nine). However, because they have much higher resistance, they cannot be charged or discharged as quickly, limiting them to low power density applications.